1. Field of the Invention
The present invention relates to a reduction gear suitable for lifters of business equipment such as copying machines.
2. Description of the Prior Art
FIGS. 4 and 5 show an apparatus used for business equipment such as copying machines. The apparatus is a combination of a reduction gear having paired involute gears with parallel axes, and a motor.
In this apparatus, the output of a motor 100 is taken from an output shaft 104 by way of five steps of involute gear rows 102A to 102E. The output characteristic is shown by the solid line in FIG. 6.
In general, the reduction mechanism used for lifters of business equipment such as copying machines requires the following functions: i) no-load rotational frequency A; ii) rating load rotational frequency B; iii) rating torque C; and iv) stall torque D, which are for example shown by black dots and broken lines in FIG. 6. The reduction mechanism further requires a reverse rotation preventive function, that is, the function of holding the position of the output shaft 104.
The above-described reduction gear using involute gear rows, however, cannot sufficiently satisfy the above functions. For example, to satisfy the function of the stall torque described in iv), that is, to satisfy the characteristic of preventing the output of a torque over a specified value for protecting a mating member, the involute reduction gear must be additionally provided with an overload preventive apparatus such as a torque limiter mechanism or overcurrent protective mechanism. Moreover, in the involute gear reduction gear, the start-up efficiency from the output side is high (positive value), and accordingly, when a load more than a specified value is applied to the output side, the output shaft is reversely rotated. Accordingly the output shaft cannot be held in its position. In this regard, to hold the position of the output shaft 104, that is, to prevent the reverse rotation of the output shaft 104, the involute reduction gear is additionally provided with a mechanism for holding the position of the output shaft or preventing the reverse rotation thereof.
As described above, the prior art reduction gear using involute gear rows is necessarily provided with additional mechanisms when used for business equipment requiring the stall torque characteristic and the function of preventing the reverse rotation. Consequently, there is a difficulty in reducing the size, weight and cost of the prior art reduction gear used for business equipment.